Conventionally, ultrapure water used as semiconductor water cleaning water is produced by treating raw water (industrial water, city water, well water, and used ultrapure water discharged from semiconductor plants (hereinafter referred to as “recovered water”)) with an ultrapure water production apparatus including a pretreatment system, primary pure water system, and subsystem (or secondary pure water system).
Ultrapure water produced with a conventional ultrapure water production apparatus contains about 2-5 μg/L of TOC (Total Organic Carbon). Methods for further reducing TOC in ultrapure water may include installation of multi-stage reverse osmosis membrane separation devices and increasing the ultraviolet irradiation dose in a low pressure ultraviolet light oxidation device, but these methods have not been successful in sufficiently reducing TOC in ultrapure water.
Patent document 1 discloses that raw water (city water, industrial water and the like) that is to become ultrapure water contains several tens to several hundreds of μg/L of urea, and that the major component of the TOC remaining in the ultrapure water is urea (NH2CONH2). Patent document 1 discloses that, in an ultrapure water production apparatus including a pretreatment device for raw water, primary pure water production device and subsystem, urea in raw water is highly removed at high levels and TOC in ultrapure water is reduced by incorporating a biological activated carbon device as a biological treatment device into the pretreatment device.
Patent document 2 discloses, in relation to the method disclosed in Patent document 1, that recovered semiconductor cleaning water is mixed with raw water and supplied to the biological activated carbon device with a view to improving the urea degradation rate.
Patent document 3 discloses that raw water (not including recovered water and recovered water) are separately treated with biological activated carbon and then supplied to the primary pure water production device to prevent the biological activated carbon treatment of urea from being inhibited by ammonium ions in the recovered semiconductor cleaning water.
Patent document 4 discloses that, in an ultrapure water production apparatus where a biological activated carbon column is installed in the primary pure water device, for the purpose of restricting the leakage of excessive fungi by adequately curbing the activities of living organisms in the biological activated carbon column and preventing the spillage of microorganisms into a downstream device, an oxidation agent such as hypochlorite is added to feed water which is supplied to the biological activated carbon column so that the residual chlorine concentration of the feed water is 0.5-5 mg/L.